Introduction
The tumor suppressor gene PTEN maps to chromosome 10q23.3, and encodes a dual specificity phosphatase. Germline mutations in PTEN are found in Cowden syndrome (CS), which is characterized by multiple hamartomas involving many organ systems as well as an increased risk of developing breast and thyroid cancers. Loss of heterozygosity (LOH) has been reported at this locus in around 40% of primary sporadic breast cancers, however only a very small fraction of these also have mutations in the remaining allele. Whether the loss of one PTEN allele is sufficient for tumorigenesis, or whether inactivation of the second allele might occur through epigenetic rather than mutational events has been open to question.